The present invention relates to a color image forming apparatus and an image forming timing control method, in which images of single colors are formed on an image forming medium and are layered sequentially to form a color image.
There are various methods for forming a color image on an image forming medium such as a paper. For example, known methods are an electronic photograph method in which electrostatic images (electrostatic latent images) are formed on a photosensitive drum and are transferred onto a paper by toner to visualize the images, an inkjet method in which ink drops are directly injected to a paper to form an image, silver salt photograph method in which an image is recorded by exposing a photosensitive coloring material, and the like.
In any of the above-described methods, images of single colors need to be layered precisely to prepare an excellent color image. If the precision of the layering is not sufficient, there appears deterioration of image quality, such as blurring of an image, insufficient sharpness, insufficient color reproductivity, and the like. In general, 0.1 mm or less is the layering precision at which dislocations in layering are not observed conspicuously with eyes but high image quality can be obtained.
Conventionally, various proposals have been made to improve the layering precision. For example, there is a technique in which a mark for detecting a dislocation in layering is formed on a conveyer belt for a recording paper and the image forming position is corrected on the basis of the detected dislocation in layering, in a color image forming apparatus having a tandem structure.
However, this technique involves problems such as increased toner consumption due to formation of a mark for detecting a layering dislocation, additional installation of a cleaning device for removing unnecessary marks after detection, which is not related with an output image, deterioration of a photosensitive material due to mechanical stress, elongated recording time due to time loss for detecting a layering dislocation, increased costs due to necessity for a detector for detecting precisely a mark, and increased running costs including totally photosensitive elements and toner.
Also, in the case of realizing a detector capable of precisely detecting a layering dislocation detection mark formed by a toner dot in order to detect a layering error between images of single colors, the detection precision of the detector itself needs to be improved to 0.02 mm or less. This results from consideration of precision allocation due to analysis of factors causing a layering error and error analysis of the structural elements of the image forming apparatus, if the layering dislocation tolerance is set to about 1 mm. At the same time, a plurality of marks must be detected to improve detection precision through statistical processing, because there are changes and variants in the driving precision of the image forming process and structural elements when forming a mark. Thus, not only hardware, including the detector, but also software, such as a detection sequence, is burdened.
On the other side, there is a technique (Japanese Patent Application KOKOKU Publication No. 4-54235) in which synchronization is carried out using one of a plurality of laser beams as a reference.
However, this technique can perform only relative positioning between the laser beams, and therefore, image forming positions determined by relative positions between the photosensitive element and the beams cannot be precisely aligned with each other. A problem hence arises in that image layering precision cannot be improved sufficiently. Also, the structure of the apparatus is complicated because a mechanism is required by which the photosensitive element, which is an expendable component and must be replaced several ten times in the lifetime of the apparatus, must be attached with sufficient precision. Further, the driving speed of the photosensitive element must be set with high precision, so that the photosensitive element must be driven by a drive transmission mechanism with high mechanical precision under highly precise speed control. As a result, a problem arises in that the apparatus costs are increased.
As a technique which solves these problems, Japanese Patent Application No. 10-332848 (corresponding to U. S. patent application Ser. No. 09/429,903) proposes a highly-precise image layering method as follows. In this method, a light reflector for reflecting a write laser beam to the surface of a photosensitive drum is provided. Four laser beams for forming respectively four images, which are reflected from the light reflector, are detected by a light detection sensor and converted into electric signals, which are used for image write timings. According to this method, four images of single colors are formed with the position of the light reflector on the photosensitive drum surface taken as a reference, so that a layered full-color image can have very high layering precision.
However, the width of laser beam scanning for forming an electrostatic latent image on the photosensitive drum surface is designed to be smaller than the width of the photosensitive layer of the photosensitive drum due to limitations such as lens size, lens manufacturing costs, and the like. Therefore, the light reflector must be set on the surface of the photosensitive layer. Although the surface of the photosensitive drum is constantly cleaned by a cleaner or the like, coloring material such as toner sticks to this surface. Consequently, the surface of the light reflector is temporarily covered with toner or the like so that the reflection rate is lowered. If reflection light from the light reflector comes without a sufficient light amount, it can be considered that images cannot be formed. Further, if a light reflector is set on the surface of the photosensitive layer, it is difficult to apply a photosensitive layer in a step of manufacturing a photosensitive drum and an insulation process is required.
Accordingly, it is an object of the present invention to provide a color image forming apparatus and an image forming timing control method, that are capable of correcting image-layering dislocations of a formed color image with high precision, without providing a light reflector on a photosensitive layer, thereby obtaining a color image with high quality at a high speed.
According to one aspect of the present invention, there is provided a color image forming apparatus comprising: a photosensitive base which is rotatable and has a photosensitive layer on a surface of the photosensitive base and a reflecting surface on the surface of the photosensitive base, the reflecting surface disposed besides the photosensitive layer; a scanning unit which scans a plurality of exposure light beams on the photosensitive base, the plurality of exposure light beams corresponding to respective latent images of different colors; a developing unit which develops the plurality of latent images; a deflecting unit which deflects the plurality of exposure light beams and guides the plurality of exposure light beams to an orbit of the reflecting surface; a detecting unit which detects the plurality of exposure light beams reflected by the reflecting surface; and a controller connected to the detecting unit and the scanning unit, and configured to determine timing of scanning of the scanning unit based on respective time points when the plurality of exposure light beams are detected.
According to another aspect of the present invention, there is provided a color image forming apparatus comprising: a photosensitive base which is rotatable, the photosensitive base comprising a photosensitive surface, a non-photosensitive surface, and a reflecting surface formed on the non-photosensitive surface; a scanning unit which scans a plurality of exposure light beams on the photosensitive base, the plurality of exposure light beams corresponding to respective latent images of different colors; a developing unit which develops the plurality of latent images; a deflecting unit which increases width of the scanning of the plurality of exposure light beams and provides the plurality of exposure light beams onto the reflecting surface; a detecting unit which detects the plurality of exposure light beams reflected by the reflecting surface; and a controller connected to the scanning unit and the detecting unit, and configured to determine respective timings of scanning of the scanning unit based on respective time points when the plurality of exposure light beams are detected.
According to still another aspect of the present invention, there is provided an image forming timing control method applied to a color image formation relating a photosensitive base, the photosensitive base comprising a photosensitive surface and a reflecting surface besides the photosensitive surface, comprising: applying a plurality of exposure light beams onto an orbit along which the reflecting surface moves; detecting the plurality of exposure light beams reflected by the reflecting surface; and determining timings of scanning of latent images on the photosensitive base, based on respective time points when the plurality of exposure light beams are detected.